Lactones and -lactams are structural motifs critical to the treatment of disease and infection whose activity is directly related to the highly strained 4-membered ring present in each. Beyond their applications in clinical settings, these compounds are valuable starting materials for the preparation of other medicinally important structures, such as -amino acids. Despite their importance, methods for the catalytic enantioselective construction of highly substituted -lactones and -lactams are limited with regard to substrate scope, catalyst design, and generality. To address these limitations, the current proposal provides a catalytic enantioselective method for the preparation of both -lactones and -lactams in a single step from simple, achiral, and readily available starting materials using a bifunctional phosphinothiourea catalyst. Specifically, the thiourea catalyst will mediate the formal [2+2] cycloaddition reaction between ketenes and aldehydes, ketones, and imines. By this process, a diverse array of products can be generated that bear vicinal stereogenic carbons. Furthermore, the application of a phosphinothiourea catalyst will allow cooperative activation of both reaction partners, forming new C-C and C-heteroatom bonds between two compounds of roughly equal complexity in a maximally convergent fashion. The application of -lactone and -lactam products generated by this method to the total synthesis of natural products and to the preparation of medicinally important derivatives will provide access to important lead compounds for the development of new pharmaceutical agents for the treatment of cancer and infectious disease.